Reproductive development in the protandrous gastropod Limax maximus is under photoperiodic control. Male-maturation (gonad enlargement and sperm production) is initiated following a transition from short- (LD 8:16) to long-day (LD 16:8) light cycles. Preliminary experiments indicate that a brain hormone which promotes male-phase gonad growth is secreted under long days. One objective of the proposed research will be to confirm this by showing that immature animals will develop when implanted with whole brains from long-day (maturing) but not from short-day (immature) donors. A second objective will be to localize the neurosecretory cells (NSC) that manufacture the hormone so that in later work their electrophysiological characteristics can be examined before and after the transition to long days. Neurosecretory stains will be used to identify NSC and partial-brain implants will be employed to locate brain region(s) involved in production of "maturation hormone". Also, as part of the proposed program the time-course of the developmental response will be characterized by monitoring 3H-thymidine incorporation into gonad DNA after beginning long-days, and the "required number" of long-day cycles for photostimulation of reproductive development will be determined. To initiate the study of photoperiod effects on brain cells, the activity and membrane properties of a large, identifiable neuron (the abdominal giant cell equals AGC) will be examined in short- and long-day animals. Differences, if found, might represent a direct effect of photoperiod on the AGC or could be determined by the presence or absence of maturation hormone. The proposed program will initiate one of the first studies of a photoperiodically-regulated molluscan neuroendocrine system controlling sexual development. Given its strong parallels with those mammalian systems in which the neuroendocrine-gonad axis is controlled by photoperiod, the Limax system is potentially an excellent model for studying photostimulated reproductive NSC at the cellular physiology level.